Loop filtering across constrained intra block boundaries in video coding

ABSTRACT

This disclosure describes techniques for coding video data. In particular, this disclosure describes techniques for loop filtering for video coding. The techniques of this disclosure may apply to loop filtering and/or partial loop filtering across block boundaries in scalable video coding processes. Loop filtering may include, for example, one or more of adaptive loop filtering (ALF), sample adaptive offset (SAO) filtering, and deblocking filtering.

This application claims the benefit of U.S. Provisional Application No.61/729,985, filed Nov. 26, 2012, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to video coding, and more particularly totechniques for loop filtering in video coding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, digital cameras, digital recording devices,digital media players, video gaming devices, video game consoles,cellular or satellite radio telephones, video teleconferencing devices,and the like. Digital video devices implement video compressiontechniques, such as those described in the standards defined by MPEG-2,MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding(AVC), the High Efficiency Video Coding (HEVC) standard presently underdevelopment, and extensions of such standards, to transmit, receive andstore digital video information more efficiently.

Video compression techniques include spatial prediction and/or temporalprediction to reduce or remove redundancy inherent in video sequences.For block-based video coding, a video frame or slice may be partitionedinto blocks. Each block can be further partitioned. Blocks in anintra-coded (I) frame or slice are encoded using spatial prediction withrespect to reference samples in neighboring blocks in the same frame orslice. Blocks in an inter-coded (P or B) frame or slice may use spatialprediction with respect to reference samples in neighboring blocks inthe same frame or slice or temporal prediction with respect to referencesamples in other reference frames. Spatial or temporal predictionresults in a predictive block for a block to be coded. Residual datarepresents pixel differences between the original block to be coded andthe predictive block.

An inter-coded block is encoded according to a motion vector that pointsto a block of reference samples forming the predictive block, and theresidual data indicating the difference between the coded block and thepredictive block. An intra-coded block is encoded according to anintra-coding mode and the residual data. For further compression, theresidual data may be transformed from the pixel domain to a transformdomain, resulting in residual transform coefficients, which then may bequantized. The quantized transform coefficients, initially arranged in atwo-dimensional array, may be scanned in a particular order to produce aone-dimensional vector of transform coefficients for entropy coding.

SUMMARY

In general, this disclosure describes techniques for coding video data.In particular, this disclosure describes techniques for loop filteringfor video coding. The techniques of this disclosure may apply to loopfiltering and/or partial loop filtering across block boundaries. Loopfiltering may include, for example, one or more of adaptive loopfiltering (ALF), sample adaptive offset (SAO) filtering, and deblockingfiltering.

In one example, the disclosure describes a method of video coding themethod comprising coding a block of video data according to a scalablevideo coding process, and limiting the application of a loop filter tothe block of video data based on characteristics of the scalable videocoding process.

In another example, the disclosure describes an apparatus configured tocode video data, the apparatus comprising a video coder configured tocode a block of video data according to a scalable video coding process,and limit the application of a loop filter to the block of video databased on characteristics of the scalable video coding process.

In another example, the disclosure describes an apparatus configured tocode video data, the apparatus comprising: means for coding a block ofvideo data according to a scalable video coding process, and means forlimiting the application of a loop filter to the block of video databased on characteristics of the scalable video coding process.

In another example, the disclosure describes a computer-readable storagemedium storing instructions that, when executed, cause one or moreprocessors of a device configured to code video data to code a block ofvideo data according to a scalable video coding process, and limit theapplication of a loop filter to the block of video data based oncharacteristics of the scalable video coding process.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system.

FIG. 2 is a conceptual diagram illustrating example edge offsetclassifications.

FIG. 3 is a conceptual diagram illustrating example band offsetclassifications.

FIG. 4 is a conceptual diagram showing region-based classification foran adaptive loop filter.

FIG. 5 is a conceptual diagram showing block-based classification for anadaptive loop filter.

FIG. 6A is a conceptual diagram showing sample adaptive offset withpadded pixels.

FIG. 6B is a conceptual diagram showing skipping a sample adaptiveoffset process.

FIG. 7 is conceptual diagram depicting a loop filter at blockboundaries.

FIG. 8 is conceptual diagram depicting asymmetric partial filters at ahorizontal boundary.

FIG. 9 is conceptual diagram depicting asymmetric partial filters at avertical boundary.

FIG. 10 is conceptual diagram depicting symmetric partial filters at ahorizontal boundary.

FIG. 11 is conceptual diagram depicting symmetric partial filters at avertical boundary.

FIG. 12 is a block diagram illustrating an example video encoder.

FIG. 13 is a block diagram illustrating an example video decoder.

FIG. 14 is a flowchart illustrating an example encoding method of thedisclosure.

FIG. 15 is a flowchart illustrating an example decoding method of thedisclosure.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for coding video data.In particular, this disclosure describes techniques for loop filteringin a video coding process.

For video coders configured to perform constrained intra prediction,pixels from neighboring blocks coded in a non-intra mode (e.g., interpredicted) may not be reconstructed. As such, pixels from neighboringblocks may be unavailable for use in loop filters (e.g., deblocking,sample adaptive offset (SAO), adaptive loop filters (ALF)). In view ofthis feature of constrained intra prediction, this disclosure presentstechniques for loop filtering, including techniques for partial loopfiltering around boundaries of intra-predicted blocks. In some examples,the loop filtering techniques of this disclosure are applied toconstrained intra-predicted blocks (i.e., blocks where someintra-prediction techniques are limited and/or constrained). Thetechniques of this disclosure may be applied to any loop filter,including deblocking, ALF, and SAO filters. In one example, thisdisclosure proposes techniques for coding a block of video dataaccording to a scalable video coding process (e.g., scalable highefficiency video coding (SHEVC)). The techniques of this disclosureinclude limiting the application of a loop filter to the block of videodata based on characteristics of the scalable video coding process.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 10 that may be configured to utilize techniques for loopfiltering in a video coding process in accordance with examples of thisdisclosure. As shown in FIG. 1, the system 10 includes a source device12 that transmits encoded video to a destination device 14 via acommunication channel 16. Encoded video data may also be stored on astorage medium 34 or a file server 36 and may be accessed by thedestination device 14 as desired. When stored to a storage medium orfile server, video encoder 20 may provide coded video data to anotherdevice, such as a network interface, a compact disc (CD), Blu-ray ordigital video disc (DVD) burner or stamping facility device, or otherdevices, for storing the coded video data to the storage medium.Likewise, a device separate from video decoder 30, such as a networkinterface, CD or DVD reader, or the like, may retrieve coded video datafrom a storage medium and provided the retrieved data to video decoder30.

The source device 12 and the destination device 14 may comprise any of awide variety of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch as so-called smartphones, televisions, cameras, display devices,digital media players, video gaming consoles, or the like. In manycases, such devices may be equipped for wireless communication. Hence,the communication channel 16 may comprise a wireless channel, a wiredchannel, or a combination of wireless and wired channels suitable fortransmission of encoded video data. Similarly, the file server 36 may beaccessed by the destination device 14 through any standard dataconnection, including an Internet connection. This may include awireless channel (e.g., a Wi-Fi connection), a wired connection (e.g.,DSL, cable modem, etc.), or a combination of both that is suitable foraccessing encoded video data stored on a file server.

Techniques for loop filtering in a video coding process, in accordancewith examples of this disclosure, may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, streaming video transmissions, e.g.,via the Internet, encoding of digital video for storage on a datastorage medium, decoding of digital video stored on a data storagemedium, or other applications. In some examples, the system 10 may beconfigured to support one-way or two-way video transmission to supportapplications such as video streaming, video playback, videobroadcasting, and/or video telephony.

In the example of FIG. 1, the source device 12 includes a video source18, a video encoder 20, a modulator/demodulator 22 and a transmitter 24.In the source device 12, the video source 18 may include a source suchas a video capture device, such as a video camera, a video archivecontaining previously captured video, a video feed interface to receivevideo from a video content provider, and/or a computer graphics systemfor generating computer graphics data as the source video, or acombination of such sources. As one example, if the video source 18 is avideo camera, the source device 12 and the destination device 14 mayform so-called camera phones or video phones. However, the techniquesdescribed in this disclosure may be applicable to video coding ingeneral, and may be applied to wireless and/or wired applications, orapplication in which encoded video data is stored on a local disk.

The captured, pre-captured, or computer-generated video may be encodedby the video encoder 20. The encoded video information may be modulatedby the modem 22 according to a communication standard, such as awireless communication protocol, and transmitted to the destinationdevice 14 via the transmitter 24. The modem 22 may include variousmixers, filters, amplifiers or other components designed for signalmodulation. The transmitter 24 may include circuits designed fortransmitting data, including amplifiers, filters, and one or moreantennas.

The captured, pre-captured, or computer-generated video that is encodedby the video encoder 20 may also be stored onto a storage medium 34 or afile server 36 for later consumption. The storage medium 34 may includeBlu-ray discs, DVDs, CD-ROMs, flash memory, or any other suitabledigital storage media for storing encoded video. The encoded videostored on the storage medium 34 may then be accessed by the destinationdevice 14 for decoding and playback.

The file server 36 may be any type of server capable of storing encodedvideo and transmitting that encoded video to the destination device 14.Example file servers include a web server (e.g., for a website), an FTPserver, network attached storage (NAS) devices, a local disk drive, orany other type of device capable of storing encoded video data andtransmitting it to a destination device. The transmission of encodedvideo data from the file server 36 may be a streaming transmission, adownload transmission, or a combination of both. The file server 36 maybe accessed by the destination device 14 through any standard dataconnection, including an Internet connection. This may include awireless channel (e.g., a Wi-Fi connection), a wired connection (e.g.,DSL, cable modem, Ethernet, USB, etc.), or a combination of both that issuitable for accessing encoded video data stored on a file server.

The destination device 14, in the example of FIG. 1, includes a receiver26, a modem 28, a video decoder 30, and a display device 32. Thereceiver 26 of the destination device 14 receives information over thechannel 16, and the modem 28 demodulates the information to produce ademodulated bitstream for the video decoder 30. The informationcommunicated over the channel 16 may include a variety of syntaxinformation generated by the video encoder 20 for use by the videodecoder 30 in decoding video data. Such syntax may also be included withthe encoded video data stored on the storage medium 34 or the fileserver 36. Each of the video encoder 20 and the video decoder 30 mayform part of a respective encoder-decoder (CODEC) that is capable ofencoding or decoding video data.

The display device 32 may be integrated with, or external to, thedestination device 14. In some examples, the destination device 14 mayinclude an integrated display device and also be configured to interfacewith an external display device. In other examples, the destinationdevice 14 may be a display device. In general, the display device 32displays the decoded video data to a user, and may comprise any of avariety of display devices such as a liquid crystal display (LCD), aplasma display, an organic light emitting diode (OLED) display, oranother type of display device.

In the example of FIG. 1, the communication channel 16 may comprise anywireless or wired communication medium, such as a radio frequency (RF)spectrum or one or more physical transmission lines, or any combinationof wireless and wired media. The communication channel 16 may form partof a packet-based network, such as a local area network, a wide-areanetwork, or a global network such as the Internet. The communicationchannel 16 generally represents any suitable communication medium, orcollection of different communication media, for transmitting video datafrom the source device 12 to the destination device 14, including anysuitable combination of wired or wireless media. The communicationchannel 16 may include routers, switches, base stations, or any otherequipment that may be useful to facilitate communication from the sourcedevice 12 to the destination device 14.

Although not shown in FIG. 1, in some aspects, the video encoder 20 andthe video decoder 30 may each be integrated with an audio encoder anddecoder, and may include appropriate MUX-DEMUX units, or other hardwareand software, to handle encoding of both audio and video in a commondata stream or separate data streams. If applicable, in some examples,MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, orother protocols such as the user datagram protocol (UDP).

The video encoder 20 and the video decoder 30 each may be implemented asany of a variety of suitable encoder circuitry, such as one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs),discrete logic, software, hardware, firmware or any combinationsthereof. When the techniques are implemented partially in software, adevice may store instructions for the software in a suitable,non-transitory computer-readable medium and execute the instructions inhardware using one or more processors to perform the techniques of thisdisclosure. Each of the video encoder 20 and the video decoder 30 may beincluded in one or more encoders or decoders, either of which may beintegrated as part of a combined encoder/decoder (CODEC) in a respectivedevice.

The video encoder 20 may implement any or all of the techniques of thisdisclosure for loop filtering in a video coding process. Likewise, thevideo decoder 30 may implement any or all of these techniques for loopfiltering in a video coding process. A video coder, as described in thisdisclosure, may refer to a video encoder or a video decoder. Similarly,a video coding unit may refer to a video encoder or a video decoder. Inthis context, a video coding unit is physical hardware and differs fromthe CU data structure discussed above. Likewise, video coding may referto video encoding or video decoding.

The video encoder 20 and the video decoder 30 may operate according to avideo compression standard, such as the HEVC and/or SHEVC standardspresently under development, and may conform to the HEVC Test Model(HM). Alternatively, the video encoder 20 and the video decoder 30 mayoperate according to other proprietary or industry standards, such asthe ITU-T H.264 standard, alternatively referred to as MPEG-4, Part 10,Advanced Video Coding (AVC), or extensions of such standards. Thetechniques of this disclosure, however, are not limited to anyparticular coding standard. Other examples include MPEG-2 and ITU-TH.263.

There is a new video coding standard, namely High-Efficiency VideoCoding (HEVC), being developed by the Joint Collaboration Team on VideoCoding (JCT-VC) of ITU-T Video Coding Experts Group (VCEG) and ISO/IECMotion Picture Experts Group (MPEG). A recent draft of the HEVCstandard, referred to as “HEVC Working Draft 9” or “WD9,” is describedin document JCTVC-K1003_v9, Bross et al., “High Efficiency video coding(HEVC) text specification draft 9,” Joint Collaborative Team on VideoCoding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, 11thMeeting: Shanghai, CN, 10-19 Oct. 2012, which, as of Oct. 18, 2013, isdownloadable fromhttp://phenix.int-evry.fr/jct/doc_end_user/documents/11_Shanghai/wg11/JCTVC-K1003-v9.zip.A more recent draft of HEVC is described in ITU-T H.265, SERIES H:AUDIOVISUAL AND MULTIMEDIA SYSTEMS, Infrastructure of AudiovisualServices—Coding of Moving Video, “High Efficiency Video Coding,” April2013 (hereinafter, “HEVC”). HEVC is incorporated by reference herein inits entirety. Various extensions to HEVC have been proposed. One suchextension is the HEVC Range Extension, described in “High EfficiencyVideo Coding (HEVC) Range Extensions text specification: Draft 4,”JCTVC-N1005_v1, April 2013 (hereinafter, “JCTVC-N1005”). A recentWorking Draft (WD) of SHEVC, entitled “High efficiency video coding(HEVC) scalable extension draft 3,” Joint Collaborative Team on VideoCoding (JCT-VC) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 14thMeeting: Vienna, AT, 25 Jul.-2 Aug. 2013, and referred to as SHEVC WD3hereinafter, is available from http://phenix.it-sudparis.eu/jct/doc_end_user/documents/14_Vienna/wg11/JCTVC-N1008-v3.zip,which is incorporated be reference herein in its entirety.

Digital video devices implement video compression techniques to encodeand decode digital video information more efficiently. Video compressionmay apply spatial (intra-frame) prediction and/or temporal (inter-frame)prediction techniques to reduce or remove redundancy inherent in videosequences.

For video coding according to the current working draft of HEVC, as oneexample, a video frame may be partitioned into coding units. A codingunit (CU) generally refers to an image region that serves as a basicunit to which various coding tools are applied for video compression. ACU usually has a luminance component, which may be denoted as Y, and twochroma components, which may be denoted as Cr and Cb. Depending on thevideo sampling format, the size of the Cr and Cb components, in terms ofnumber of samples, may be the same as or different from the size of theY component. A CU is typically square, and may be considered to besimilar to a so-called macroblock, e.g., under other video codingstandards such as ITU-T H.264.

To achieve better coding efficiency, a coding unit may have variablesizes depending on video content. In addition, a coding unit may besplit into smaller blocks for prediction or transform. In particular,each coding unit may be further partitioned into prediction units (PUs)and transform units (TUs). Prediction units may be considered to besimilar to so-called partitions under other video coding standards, suchas H.264. Transform units (TUs) refer to blocks of residual data towhich a transform is applied to produce transform coefficients.

Coding according to some of the presently proposed aspects of thedeveloping HEVC standard will be described in this application forpurposes of illustration. However, the techniques described in thisdisclosure may be useful for other video coding processes, such as thosedefined according to H.264 or other standard or proprietary video codingprocesses.

HEVC standardization efforts are based on a model of a video codingdevice referred to as the HEVC Test Model (HM). The HM presumes severalcapabilities of video coding devices over devices according to, e.g.,ITU-T H.264/AVC. For example, whereas H.264 provides nineintra-prediction encoding modes, HM provides as many as thirty-fiveintra-prediction encoding modes.

According to the HM, a CU may include one or more prediction units (PUs)and/or one or more transform units (TUs). Syntax data within a bitstreammay define a largest coding unit (LCU), which is a largest CU in termsof the number of pixels. In general, a CU has a similar purpose to amacroblock of H.264, except that a CU does not have a size distinction.Thus, a CU may be split into sub-CUs. In general, references in thisdisclosure to a CU may refer to a largest coding unit of a picture or asub-CU of an LCU. An LCU may be split into sub-CUs, and each sub-CU maybe further split into sub-CUs. Syntax data for a bitstream may define amaximum number of times an LCU may be split, referred to as CU depth.Accordingly, a bitstream may also define a smallest coding unit (SCU).This disclosure also uses the term “block”, “partition,” or “portion” torefer to any of a CU, PU, or TU. In general, “portion” may refer to anysub-set of a video frame.

An LCU may be associated with a quadtree data structure. In general, aquadtree data structure includes one node per CU, where a root nodecorresponds to the LCU. If a CU is split into four sub-CUs, the nodecorresponding to the CU includes four leaf nodes, each of whichcorresponds to one of the sub-CUs. Each node of the quadtree datastructure may provide syntax data for the corresponding CU. For example,a node in the quadtree may include a split flag, indicating whether theCU corresponding to the node is split into sub-CUs. Syntax elements fora CU may be defined recursively, and may depend on whether the CU issplit into sub-CUs. If a CU is not split further, it is referred as aleaf-CU.

Moreover, TUs of leaf-CUs may also be associated with respectivequadtree data structures. That is, a leaf-CU may include a quadtreeindicating how the leaf-CU is partitioned into TUs. This disclosurerefers to the quadtree indicating how an LCU is partitioned as a CUquadtree and the quadtree indicating how a leaf-CU is partitioned intoTUs as a TU quadtree. The root node of a TU quadtree generallycorresponds to a leaf-CU, while the root node of a CU quadtree generallycorresponds to an LCU. TUs of the TU quadtree that are not split arereferred to as leaf-TUs.

A leaf-CU may include one or more prediction units (PUs). In general, aPU represents all or a portion of the corresponding CU, and may includedata for retrieving a reference sample for the PU. For example, when thePU is inter-mode encoded, the PU may include data defining a motionvector for the PU. The data defining the motion vector may describe, forexample, a horizontal component of the motion vector, a verticalcomponent of the motion vector, a resolution for the motion vector(e.g., one-quarter pixel precision or one-eighth pixel precision), areference frame to which the motion vector points, and/or a referencelist (e.g., list 0 or list 1) for the motion vector. Data for theleaf-CU defining the PU(s) may also describe, for example, partitioningof the CU into one or more PUs. Partitioning modes may differ dependingon whether the CU is not predictively coded, intra-prediction modeencoded, or inter-prediction mode encoded. For intra coding, a PU may betreated the same as a leaf transform unit described below.

To code a block (e.g., a prediction unit (PU) of video data), apredictor for the block is first derived. The predictor can be derivedeither through intra (I) prediction (i.e. spatial prediction) or inter(P or B) prediction (i.e. temporal prediction). Hence, some predictionunits may be intra-coded (I) using spatial prediction with respect toneighboring reference blocks in the same frame, and other predictionunits may be inter-coded (P or B) with respect to reference blocks inother frames. The reference blocks used for prediction may includeactual pixel values at so-called integer pixel positions as referencesamples, or synthesized pixel values produced by interpolation atfractional pixel positions as reference samples.

Upon identification of a predictor, the difference between the originalvideo data block and its predictor is calculated. This difference isalso called the prediction residual, and refers to the pixel differencesbetween the pixels of the block to be coded and corresponding referencesamples (which may be integer-precision pixels or interpolatedfractional-precision pixels, as mentioned above) of the reference block,i.e., predictor. To achieve better compression, the prediction residual(i.e., the array of pixel difference values) is generally transformedfrom the pixel (i.e., spatial) domain to a transform domain, e.g., usinga discrete cosine transform (DCT), integer transform, Karhunen-Loeve(K-L) transform, or other transform. The transform domain may be, forexample, a frequency domain.

Coding a PU using inter-prediction involves calculating a motion vectorbetween a current block and a block in a reference frame. Motion vectorsare calculated through a process called motion estimation (or motionsearch). A motion vector, for example, may indicate the displacement ofa prediction unit in a current frame relative to a reference sample of areference frame. A reference sample may be a block that is found toclosely match the portion of the CU including the PU being coded interms of pixel difference, which may be determined by sum of absolutedifference (SAD), sum of squared difference (SSD), or other differencemetrics. The reference sample may occur anywhere within a referenceframe or reference slice. In some examples, the reference sample may beinterpolated, in whole or in part, and occur at a fractional pixelposition. Upon finding a portion of the reference frame that bestmatches the current portion, the encoder determines the current motionvector for the current portion as the difference in the location fromthe current portion to the matching portion in the reference frame(e.g., from the center of the current portion to the center of thematching portion).

In some examples, an encoder may signal the motion vector for eachportion in the encoded video bitstream. The signaled motion vector isused by the decoder to perform motion compensation in order to decodethe video data. However, signaling the original motion vector directlymay result in less efficient coding, as a large number of bits aretypically needed to convey the information.

Once motion estimation is performed to determine a motion vector for acurrent portion, the encoder compares the matching portion in thereference frame to the current portion. This comparison typicallyinvolves subtracting the portion (which is commonly referred to as a“reference sample”) in the reference frame from the current portion andresults in so-called residual data, as mentioned above. The residualdata indicates pixel difference values between the current portion andthe reference sample. The encoder then transforms this residual datafrom the spatial domain to a transform domain, such as the frequencydomain. Usually, the encoder applies a discrete cosine transform (DCT)to the residual data to accomplish this transformation. The encoderperforms this transformation in order to facilitate the compression ofthe residual data because the resulting transform coefficients representdifferent frequencies, wherein the majority of energy is usuallyconcentrated on a few low frequency coefficients.

Typically, the resulting transform coefficients are grouped together ina manner that enables entropy coding, especially if the transformcoefficients are first quantized (rounded). The encoder then performsstatistical lossless (or so-called “entropy”) encoding to furthercompress the run-length coded quantized transform coefficients. Afterperforming lossless entropy coding, the encoder generates a bitstreamthat includes the encoded video data.

The video encoding process may also include a so-called “reconstructionloop” whereby encoded video blocks are decoded and stored in a referenceframe buffer for use as reference frames for subsequently coded videoblocks. The reference frame buffer also is referred to as the decodedpicture buffer or DPB. The reconstructed video blocks are often filteredbefore storing in the reference frame buffer. Filtering is commonlyused, for example, to reduce blockiness or other artifacts common toblock-based video coding. Filter coefficients (sometimes called filtertaps) may be defined or selected in order to promote desirable levels ofvideo block filtering that can reduce blockiness and/or improve thevideo quality in other ways. A set of filter coefficients, for example,may define how filtering is applied along edges of video blocks or otherlocations within video blocks. Different filter coefficients may causedifferent levels of filtering with respect to different pixels of thevideo blocks. Filtering, for example, may smooth or sharpen differencesin intensity of adjacent pixel values in order to help eliminateunwanted artifacts.

As one example, a deblocking filter may be used to improve theappearance (e.g., smooth the edges) between blocks of coded video data.Another example filter is a sample adaptive offset (SAO) filter that isused to add offset to reconstructed blocks of pixels to improve imagequality and coding efficiency. Another type of filter that is used inthe reconstruction loop in one proposal for HEVC is the adaptive loopfilter (ALF). The ALF is typically performed after a deblocking filter.The ALF restores the fidelity of pixels degraded by the video codingcompression process. The ALF attempts to minimize the mean squared errorbetween the original pixel values in the source frame and those of thereconstructed frame. An ALF may also be applied at the output of a videodecoder in the same fashion as was applied during the encoding process.Collectively, any filter used in the reconstruction loop may be referredto as a “loop filter.” Loop filters may include one or more deblockingfilters, SAO filters, and ALFs. In addition, other types of filters foruse in the reconstruction loop are also possible.

The above-described HEVC techniques may also be used to code two or more“layers” in the emerging scalable extensions of HEVC (SHEVC). In SHEVC,a base layer and one or more enhancement layers may be coded in mannersuch that the base layer is independently coded from the one or moreenhancement layers. Video coded in the base layer provides a first (orbase) level of video resolution. Additional enhancement layers may alsobe coded in manner that, when added to the output of the base layer, mayproduce a higher resolution video. SHEVC may be particularly applicablefor situations where lower levels of bandwidth may be available fortransmission of video. In some circumstances, only the base layer may beable to be transmitted. When additional bandwidth is available, one ormore enhancement layers may also be transmitted. In this way, streamingvideo connectivity may be maintained by temporarily lowering the qualityof the video.

In some proposals for HEVC, the deblocking (DB) and sample adaptiveoffset (SAO) filters are employed. In some proposals for SHEVC, DB andSAO are used in the base layer, while DB, SAO and adaptive loop filters(ALF) are considered for use in the enhancement layer. Furthermore, SAOand ALF are considered for use in the inter-layer prediction path (i.e.,inter-prediction performed between two layers).

In some example SAO implementations for HEVC and SHEVC, a partition(which consists of a set of LCUs) can have one of three pixelclassifications: no offset, edge classification based type, and bandclassification based offset type. Further, the edge classification basedtype includes four edge offset classifications: one dimensional (1D)0-degree edge (also referred to as SAO edge offset of classificationzero or SAO_EO_(—)0), 1D 90-degree edge (also referred to as SAO edgeoffset of classification one or SAO_EO_(—)1), 1D 135-degree edge (alsoreferred to as SAO edge offset of classification two or SAO_EO_(—)2),and 1D 45-degree edge (also referred to as SAO edge offset ofclassification three or SAO_(—) EO_(—)3). Band classification basedoffset type includes two band offset type classifications: central bandand side band.

An edge classification based type SAO technique classifies each pixelwithin a partition based on edge information. FIG. 2 is a conceptualdiagram showing four possible edge offset classifications. JCT-VC E049(“CE13: Sample adaptive offset with LCU-independent decoding,” Fu etal., Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3and ISO/IEC JTC1/SC29/WG11, 5th Meeting: Geneva, CH, 16-23 Mar., 2011)describes one example of an edge classification technique that includesthe four edge offset type classifications described above. For a givenedge classification shown in FIG. 2, an edge type for the current pixelis calculated by comparing the value of the current pixel (C) to thevalues of neighboring pixels (1 and 2). In some examples, pixel valuesmay be an 8-bit string including 256 possible values or a 10-bit stringincluding 1024 possible values. For SAO_EO_(—)0 (0 degrees), the currentpixel is compared to the left and right neighbor pixels. For SAO_EO_(—)1(90 degrees), the current pixel is compared to the top and bottomneighbor pixels. For SAO_EO_(—)2 (135 degrees), the current pixel iscompared to the upper left and bottom right neighbor pixels. ForSAO_EO_(—)3 (45 degrees), the current pixel is compared to the bottomleft and upper right neighbor pixels.

Initially, the edge type of the current pixel is assumed to be zero. Ifthe value of current pixel C is equal to values of both the left andright neighbor pixels (1 and 2), the edge type remains at zero. If thevalue of the current pixel C is greater than the value of neighbor pixel1, the edge type is increased by one. If the value of the current pixelC is less than the value of neighbor pixel 1, the edge type is decreasedby one. Likewise, if the value of the current pixel C is greater thanthe value of neighbor pixel 2, the edge type is increased by one, and ifthe value of the current pixel C is less than the value of the neighborpixel 2, the edge type is decreased by 1.

As such, the current pixel C may have an edge type of either −2, −1, 0,1, or 2, where (1) the edge type is −2 if the value of current pixel Cis less than both values of neighbor pixels 1 and 2; (2) the edge typeis −1 if the value of current pixel C is less than one neighbor pixel,but equal to the other neighbor pixel; (3) the edge type is 0 if thevalue of current pixel C is the same as both neighbor pixels, or if thevalue of current pixel C is greater than one neighbor pixel, but lessthan the other neighbor pixel; (4) the edge type is 1 if the value ofthe current pixel C is greater than one neighbor pixel, but equal to theother neighbor pixel; and (5) the edge type is 2 if the value of thecurrent pixel C is greater than both values of neighbor pixels 1 and 2.It should be noted that when one of neighboring pixels 1 and 2 is notavailable (i.e., current pixel C is located at the edge of a frame orpartition), a default edge type may be defined.

In view of the above description, for each edge offset classification,edge type values may be computed with the following equations:

EdgeType=0;

if (C>Pixel 1)EdgeType=EdgeType+1;

if (C<Pixel 1)EdgeType=EdgeType−1;

if (C>Pixel 2)EdgeType=EdgeType+1;

if (C<Pixel 2)EdgeType=EdgeType−1;

Once an edge type is determined for a current pixel an offset value canbe determined for the current pixel. Offset values are based on thedifference between the original video frame and the reconstructed videoframe. In one example, each non-zero edge type value (i.e., −2, −1, 1,and 2) has one offset value calculated by taking an average ofdifferences between the values of original and reconstructed pixelsbelonging to each category in a partition. The four offset values may bedenoted as eoffset⁻², eoffset⁻¹, eoffset₁, and eoffset₂. Because each ofeoffset⁻², eoffset⁻¹, eoffset₁, and eoffset₂ is based on the originalvideo frame, which is not available at a video decoder, a video decoderincludes a mechanism to derive the four offset values without relying onthe original video frame.

Band classification based offset type classification classifies pixelsinto different bands based on their intensity. As described above, bandclassification based offset type classification may include two bandoffset type classifications: central band and side band. FIG. 3 is aconceptual diagram showing an example band classification based offsettype classification including a central band and a side band. As shownin FIG. 3, each of pixel intensities 0 to MAX may be categorized intoone of 32 bands. In one example, pixels may have 8-bit intensity valuesand MAX may equal 255. In the example of FIG. 3, the 16 bands in thecenter are classified into a first group and the remaining side bandsare classified into a second group. In a manner similar to edge typeoffset classification, once a band type is determined for a currentpixel, an offset value can be determined for the current pixel based onthe difference between the original video frame and the reconstructedvideo frame. In one example, each band type value (i.e., 0 to 31) hasone offset value calculated by taking an average of differences betweenthe values of original and reconstructed pixels belonging to each bandtype category in a partition. Thus, for each group of bands (i.e., firstgroup and second group), 16 offset values are determined. The 16 offsetvalues for each group may be denoted as boffset₀, . . . , boffset₁₅. Aswith eoffset⁻², eoffset ⁻¹, eoffset₁, and eoffset₂, each of boffset₀, .. . , boffset₁₅ is based on the original video frame and a video decoderincludes a mechanism to derive the 16 offset values.

Typically, an encoded video bitstream includes information indicatingone of the six pixel classification types and a corresponding set ofoffsets (i.e., eoffset⁻², . . . , eoffset₂ and boffset₀, . . . ,boffset₁₅) for each partition of a video frame. In some cases, eachoffset value in a set of offset values is independently coded usingsigned unary coding on a partition-by-partition basis. Independentlycoding offset values using signed unary coding fails to exploit possiblecorrelations between offset values within a set offset values or betweenoffset values of neighboring partitions (or partitions from previousframes). Thus, independently coding offset values using signed unarycoding may not provide the most efficient bit rate.

Further, SAO techniques may be executed in conjunction with additionalfiltering techniques. Additional filtering techniques may include, forexample, Weiner filtering techniques. Similar to the calculation ofoffset values for SAO techniques, additional filtering techniques maycalculate filter coefficients based on the difference between theoriginal frame and the reconstructed frame. For example, filtercoefficients for a Weiner filter may be determined based on thedifference between the original picture and a reconstructed picture.Like offset values, calculated coefficients may also be included in thebitstream for use by a video decoder.

In one example ALF process proposed for HEVC and SHEVC, a filtered pixelvalue is derived by taking a summation of adjusted values of current andneighboring pixels within a partition of a video block, wherein thevalues of current and neighboring pixels are adjusted by multiplyingcalculated AC coefficients and adding DC coefficients to the current andneighboring pixels. The value of the summation may further be normalizedby dividing the result of the summation by the total number of pixelsincluded in partition. The equation below provides an example equationfor calculating a filtered pixel using AC and DC coefficients, whereinthe pixel is included in partition of size 1 by m and bit_shift is anormalizing factor:

Filtered pixel (x,y)=(sum_(l,m)(prefiltered pixel(x+1,y+m)*ACcoefficients(l,m))+DC coefficients)>>bit_shift.

In one ALF proposal for HEVC and SHEVC, two adaptation modes (i.e.,block and region adaptation modes) are proposed. For region adaptivemode, a frame is divided into 16 regions, and each region can have oneset of linear filter coefficients (a plurality of AC coefficients andone DC coefficient), and one region can share the same filtercoefficients with other regions. FIG. 4 is a conceptual diagram showingregion-based classification for an adaptive loop filter. As shown inFIG. 4, frame 120 is divided into 16 regions, and each region mayinclude multiple CUs. Each of these 16 regions is represented by anumber (0-15) that indicates the particular set of linear filtercoefficients used by that region. The numbers (0-15) may be indexnumbers to a predetermined set of filter coefficients that are stored atboth a video encoder and a video decoder. In one example, a videoencoder may signal, in the encoded video bitstream, the index number ofthe set of filter coefficients used by the video encoder for aparticular region. Based on the signaled index, a video decoder mayretrieve the same predetermined set of filter coefficients to use in thedecoding process for that region. In other examples, the filtercoefficients are signaled explicitly for each region.

For a block-based classification mode, a frame is divided into 4×4blocks, and each 4×4 block derives one class by computing a metric usingdirection and activity information. For each class, one set of linearfilter coefficients (a plurality of AC coefficients and one DCcoefficient) can be used and one class can share the same filtercoefficients with other classes. FIG. 5 is a conceptual diagram showingblock-based classification for an adaptive loop filter.

The computation of the direction and activity, and the resulting metricbased on direction and activity, are shown below:

Direction

Ver_act(i,j)=abs(X(i,j)<<1−X(i,j−1)−X(i,j+1))

Hor_act(i,j)=abs(X(i,j)<<1−X(i−1,j)−X(i+1,j))

H _(B)=Σ_(i−0,2)Σ_(j−0,2) H(i,j)

V _(B)=Σ_(i=0,2)Σ_(j=0,2) H(i,j)

Direction=0, 1(H>2V), 2 (V>2H)

Activity

L _(B) =H _(B) +V _(B)

5 classes (0, 1, 2, 3, 4)

Metric

Activity+5*Direction

Hor_act (i, j) generally refers to the horizontal activity of currentpixel (i, j), and Vert_act(i, j) generally refers to the verticalactivity of current pixel (i,j). X(i, j) generally refers to a pixelvale of pixel (i, j)), where i and j indicate horizontal and verticalcoordinates of the current pixel. In this context, activity is generallythe gradient or variance among pixels in a location.

H_(B) refers to the horizontal activity of the 4×4 block, which in thisexample is determined based on a sum of horizontal activity for pixels(0, 0), (0, 2), (2, 0), and (2, 2). V_(B) refers to the verticalactivity of the 4×4 block, which in this example is determined based ona sum of vertical activity for pixels (0, 0), (0, 2), (2, 0), and (2,2). “<<1” represents a multiply by two operation. Based on the values ofH_(B) and V_(B), a direction can be determined.

As one example, if the value of H_(B) is more than 2 times the value ofV_(B), then the direction can be determined to be direction 1 (i.e.,horizontal), which might correspond to more horizontal activity thanvertical activity. If the value of V_(B) is more than 2 times the valueof H_(B), then the direction can be determined to be direction 2 (i.e.,vertical), which might correspond to more vertical activity thanhorizontal activity. Otherwise, the direction can be determined to bedirection 0 (i.e., no direction), meaning neither horizontal norvertical activity is dominant. The labels for the various directions andthe ratios used to determine the directions merely constitute oneexample, as other labels and ratios can also be used.

Activity (L_(B)) for the 4×4 block can be determined as a sum of thehorizontal and vertical activity. The value of L_(B) can be classifiedinto a range. This particular example shows five ranges, although moreor fewer ranges may similarly be used. Based on the combination ofactivity and direction, a filter for the 4×4 block of pixels can beselected. As one example, a filter may be selected based on atwo-dimensional mapping of activity and direction to filters, oractivity and direction may be combined into a single metric, and thatsingle metric may be used to select a filter (e.g., themetric=Activity+5*Direction).

Returning to FIG. 5, block 140 represents a 4×4 block of pixels. In thisexample, only four of the sixteen pixels are used to calculate activityand direction metrics for a block-based ALF. The four pixels are pixel(0, 0) which is labeled as pixel 141, pixel (2, 0) which is labeled aspixel 142, pixel (0, 2) which is labeled as pixel 143, and pixel (2, 2)which is labeled as pixel 144. The Horizontal activity of pixel 141(i.e., hor_act(0, 0)), for example, is determined based on a leftneighboring pixel and a right neighboring pixel. The right neighboringpixel is labeled as pixel 145. The left neighboring pixel is located ina different block than the 4×4 block and is not shown in FIG. 5. Thevertical activity of pixel 142 (i.e., ver_act(2, 0)), for example, isdetermined based on an upper neighboring pixel and a lower neighboringpixel. The lower neighboring pixel is labeled as pixel 146, and theupper neighboring pixel is located in a different block than the 4×4block and is not shown in FIG. 5. Horizontal and vertical activity maybe calculated for pixels 143 and 144 in a similar manner.

When single-loop decoding of SHEVC is performed, inter-layer blockprediction (IntraBL) uses reconstructed pixels from the base layer whoseblock type is intra (i.e., the block was intra-predicted). In oneproposal for SHEVC, the intra-prediction performed on such base layerblocks is always constrained intra prediction. Constrained intraprediction is a loss-resilience feature in HEVC. Neighboring referencesamples inside any inter-predicted block are considered not available inorder to avoid letting potentially corrupted prior decoded picture datapropagate errors into the prediction signal. As such, in constrainedintra prediction, when performing intra-prediction on a current block,neighboring blocks that were inter-predicted are not allowed to be usedas predictors. This constraint allows only one motion compensation inthe target layer.

For this purpose, previous scalable video coding (SVC) techniques (e.g.,the scalable video extension of H.264/AVC) used a special treatment fordeblocking operations. Deblocking between inter and intra blocks of baselayer is not allowed, because allowance of deblocking between inter andintra blocks requires reconstruction of neighboring inter blocks (i.e.,in H.264-AVC/SVC, deblock strength is set to 0 ifinterLayerDeblockingFlag is equal to 1 and mbType[mbAddrP ] is Intermacroblock prediction mode).

Now, in SHEVC, there are other loop filters, such as SAO and ALF, whichmay benefit from special handling when those filters are applied toconstrained intra blocks in the base layer. Also, the same problems mayoccur when loop filters are used in inter-layer prediction (e.g.,inter-layer SAO and ALF).

In view of these potential drawbacks, this disclosure presentstechniques for loop filtering, including techniques for partial loopfiltering around boundaries of intra-predicted blocks. In some examples,the loop filtering techniques of this disclosure are applied toconstrained intra-predicted blocks (i.e., blocks where someintra-prediction techniques are limited and/or constrained). Thetechniques of this disclosure may be applied to any loop filter,including deblocking, ALF, and SAO filters. In one example, thisdisclosure proposes techniques for coding a block of video dataaccording to a scalable video coding process (e.g., SHEVC). Thetechniques of this disclosure include limiting the application of a loopfilter to the block of video data based on characteristics of thescalable video coding process.

In this context, limiting the application of a loop filter involvesperforming a loop filtering process that is in some way different orlimited in relation to the loop filter processes specified in proposalsfor HEVC and SHEVC. The examples of limiting the application of a loopfilter described below may include the restriction of what pixels areused as an input to a filter process, what pixels are actually filtered,and not performing filtering at all based on certain characteristics ofthe scalable video coding process (e.g., characteristics of SHEVC).Example characteristics that may be used to determine how to limit loopfiltering processes may include the use or non-use of constrainedintra-coding, the use of inter-layer prediction, and whether or not theblock to be filtered is in a base or enhancement layer.

The next section will discuss the techniques of this disclosure for SAOcoding in a base layer. In one example of the disclosure, whendetermining whether or not to apply SAO filtering at the boundary ofconstrained intra blocks of the base layer, several solutions arepresented to help the use of pixels from non-intra neighboring blocks.As described above, because non-intra neighboring blocks may not bereconstructed when using constrained intra-prediction for a block ofvideo data, pixels for non-intra neighboring blocks may not be availablefor use in loop filtering processes, including SAO filtering. Inparticular, pixels from neighboring blocks may be unavailable to makeedge offset or band offset determinations for SAO.

In one example of the disclosure for SAO coding in the base layer onconstrained intra blocks, rather than using the pixels of non-intraneighboring blocks to apply the SAO, padded pixels are used instead.That is, rather than using pixels from non-intra neighboring blocks tomake a determination of edge offset type, as well as the offsets to beused, pixels from the constrained intra-block are reused (i.e., padded)in place of any unavailable pixels needed from neighboring non-intrablocks. Then the edge offset type, as well as the offset valuesthemselves, may be determined as normal, as was outlined above withreference to FIG. 2. FIG. 6A shows an example for edge offset type zero(SAO_EO_(—)0) classification where a pixel (pixel P) is unavailable asit is located in a non-intra neighboring block. In the example of FIG.6A, a padded pixel is used in place of the unavailable pixel. Note thatsimilar techniques may be applied to the other edge offset types.

In another example of the disclosure for SAO coding in the base layer onconstrained intra blocks, SAO coding is skipped for boundary pixels inthe case that the neighboring block is not intra-coded. That is, SAOfiltering is not applied for pixels at boundaries of a constrained intrablock in situations where pixels from an unavailable non-intraneighboring block are needed for making an edge offset typedetermination, and/or are needed for the determination of the offsetvalues themselves. FIG. 6B shows an example for edge offset type zero(SAO_EO_(—)0) classification where a pixel (pixel X) is unavailable asit is located in a non-intra neighboring block. In the example of FIG.6B, SAO is skipped for pixel C. Note that similar techniques may beapplied to the other edge offset types. In one example, SAO coding maybe skipped for an entire block where one pixel in the block uses a pixelfrom a non-intra neighboring block to make an edge offset typedetermination. In another example, as will be discussed below, SAOcoding may be skipped only for the pixel that uses a pixel from anon-intra neighboring block.

In another example of SAO coding in the base layer on constrained intrablocks, SAO coding may be selectively skipped depending on the sao_typeon the boundary pixels. For example, when band offset (BO) is used, nopixels are skipped because BO only requires the current pixel (i.e., apixel in the intra-coded block) for classification. If edge offset (EO)is used, SAO is skipped only on pixel positions which require access topixels belonging to neighboring inter blocks (i.e., neighbor blocks thatare not intra-coded as shown in FIG. 6B).

In another example of the disclosure for SAO coding in the base layer onconstrained intra blocks, SAO coding is skipped or the SAO type is setto zero (meaning SAO is not applied) for all constrained intra blocks ofa base layer, regardless of whether or not pixels from neighboringblocks are needed to make an edge offset type determination. In anotherexample of the disclosure for SAO coding in the base layer onconstrained intra blocks, SAO coding is skipped or the SAO type is setto zero (meaning SAO is not applied) when at least one of theneighboring blocks is not intra-coded.

In another example of the disclosure for SAO coding in the base layer onconstrained intra blocks, avoiding usage of non-intra pixels (e.g.,pixels from neighboring inter-predicted blocks) in the SAO process maybe accomplished through encoder restriction and imposing a bitstreamconstraint where the bitstream shall not contain the SAO type thatrequires non-intra pixels for the SAO process, in particular for pixelclassification. That is, rather than requiring video decoder 30 to makea determination of whether or not to apply SAO to particular block,video encoder 20 may be configured such that it does not signal SAOtypes that may require special handling at boundaries of constrainedintra blocks (e.g., edge offset types).

In addition to all the solutions mentioned above for SAO coding in abase layer, video encoder 20 may be configured to signal a flag (flag1)in the encoded video bitstream to indicate that SAO application isrestricted to intra pixels in the base layer.

The next section of the disclosure will discuss SAO coding for interlayer prediction. In one example of the disclosure for SAO coding ininter layer prediction, rather than using the pixels of non-intraneighboring blocks to apply the SAO (e.g., for determining SAO edgeoffset type), padded pixels are used instead. The same techniques asdescribed above with reference to FIG. 6A may be used. In one example,SAO coding may be skipped for an entire block where one pixel in theblock uses a pixel from a non-intra neighboring block to make an edgeoffset type determination. In another example, as will be discussedbelow, SAO coding may be skipped only for the pixel that uses a pixelfrom a non-intra neighboring block.

In another example of the disclosure for SAO coding in inter layerprediction, SAO coding is skipped for boundary pixels where neighboringblocks are not intra-coded. The same techniques as described above withreference to FIG. 6B may be used.

In another example of the disclosure for SAO coding in inter layerprediction, SAO coding is selectively skipped depending on the sao_typeof the boundary pixels in the case that a neighboring block is notintra-coded. For example, when band offset (BO) is used, no pixels areskipped because BO only requires a current pixel for classification. Ifedge offset (EO) is used, SAO is skipped only on pixel positions whichrequire access to pixels belonging to inter blocks. In another exampleof SAO coding in inter layer prediction, SAO coding is skipped or SAOtype is set to zero (meaning SAO is not applied) for constrained intrablocks of base layer.

In another example of SAO coding in inter layer prediction, SAO codingis skipped or SAO type is set to zero (meaning SAO is not applied) forconstrained intra blocks of base layer when at least one of neighboringblock is not intra-coded.

In another example of SAO coding in inter layer prediction, avoidingusage of non-intra pixels in the SAO process may be accomplished throughencoder restriction and imposing a bitstream constraint where thebitstream shall not contain the SAO type that requires non-intra pixelsfor the SAO process, in particular for pixel classification.

In addition to all the solutions mentioned above, a video encoder maysignal, and a video decoder may receive, a flag (flag2) in the encodedvideo bitstream to indicate that SAO application is restricted to intrapixels in the base layer.

The next section will discuss ALF coding for inter layer prediction.When inter-layer ALF is used in the boundary of constrained intrablocks, there are several solutions possible to prevent the use ofpixels from non-intra neighboring blocks. As discussed above, forconstrained intra blocks/frames, pixels from non-intra neighboringblocks may not be reconstructed, and thus may be unavailable for usewith ALF.

In one example of ALF coding in inter layer prediction, rather thanusing the pixels of non-intra neighboring blocks to apply the ALF,padded pixels are used instead. In another example of ALF coding ininter layer prediction, ALF coding is skipped on boundary pixels whereneighboring blocks are not intra-coded. Similar to the examples abovewith regard to FIGS. 6A and 6B, if the filter mask for ALF extends topixels in neighboring non-intra blocks, ALF filtering for such boundarypixels may be skipped, or padded data may be used. In another example ofALF coding in inter layer prediction, ALF coding is skipped forconstrained intra blocks of the base layer. In another example of ALFcoding in inter layer prediction, ALF coding is skipped for constrainedintra blocks of the base layer when at least one of the neighboringblocks is not intra-coded.

As such, in one example, ALF coding may be skipped for an entire blockwhere one pixel in the block uses a pixel from a non-intra neighboringblock to make an edge offset type determination. In another example, ALFcoding may be skipped only for the pixel that uses a pixel from anon-intra neighboring block.

In another example of ALF coding in inter layer prediction, partial ALFfiltering is used. In partial ALF filtering, only the available pixelsfor the filter are used (i.e., filter taps whose corresponding pixelsare not available are not used). Partial filtering may be symmetrical orasymmetrical. Symmetrical and asymmetrical partial filtering isdiscussed in more detail below with reference to FIGS. 7-11.

FIG. 7 is conceptual diagram depicting an ALF at block boundaries.Horizontal block boundary 201 is depicted as a horizontal line andvertical block boundary 202 is depicted as a vertical line. The filledcircles (i.e., dots) of filter mask 200 in FIG. 7 represent coefficients(i.e., weights) of the filter, which are applied to pixels of thereconstructed video block in the slice and/or tile. That is, the valueof a coefficient of the filter may be applied to the value of acorresponding pixel, such that the value of the corresponding ismultiplied by the coefficient value to produce a weighted pixel value.The pixel value may include a luminance value and one or morechrominance values. Assuming that the center of the filter is positionedat the position of (or in close proximity to) the pixel to be filtered,a filter coefficient may be said to correspond to a pixel that iscollocated with the position of the coefficient. Pixels corresponding tocoefficients of a filter can also be referred to as “supporting pixels”or collectively, as a “set of support” for the filter. The filteredvalue of a current pixel 203 (corresponding to the center pixel maskcoefficient C0) is calculated my multiplying each coefficient in filtermask 200 by the value of its corresponding pixel, and summing eachresulting value.

In this disclosure, the term “filter” generally refers to a set offilter coefficients. For example, a 3×3 filter may be defined by a setof 9 filter coefficients, a 5×5 filter may be defined by a set of 25filter coefficients, a 9×5 filter may be defined by a set of 45 filtercoefficients, and so on. Filter mask 200 shown in FIG. 7 is a 7×5 filterhaving 7 filter coefficients in the horizontal direction and 5 filtercoefficients in the vertical direction (the center filter coefficientcounting for each direction). However, any number of filter coefficientsmay be applicable for the techniques of this disclosure. The term “setof filters” generally refers to a group of more than one filter. Forexample, a set of two 3×3 filters, could include a first set of 9 filtercoefficients and a second set of 9 filter coefficients. The term“shape,” sometimes called the “filter support,” generally refers to thenumber of rows of filter coefficients and number of columns of filtercoefficients for a particular filter. For example, 9×9 is an example ofa first shape, 7×5 is an example of a second shape, and 5×9 is anexample of a third shape. In some instances, filters may takenon-rectangular shapes including diamond-shapes, diamond-like shapes,circular shapes, circular-like shapes, hexagonal shapes, octagonalshapes, cross shapes, X-shapes, T-shapes, other geometric shapes, ornumerous other shapes or configuration. The example in FIG. 7 is a crossshape; however other shape may be used. In most common cases, regardlessof the shape of the filter, the center pixel in the filter mask is theone that is being filtered. In other examples, the filter pixel isoffset from the center of the filter mask.

In accordance with the example discussed above, ALF may be disabled orlimited across block boundaries. In one example, this disclosureproposes using partial filters around block boundaries. A partial filteris a filter that does not use one or more filter coefficients that aretypically used for the filtering process. In one example, thisdisclosure proposes using partial filters where at least the filtercoefficients corresponding to pixels on the other side of a blockboundary are not used. Hence, in some examples, there is no need toprovide padded data for the pixels on the other side of the blockboundary. Rather, a partial filter can be configured to omit the pixelson the other side of the block boundary.

In one example, asymmetric partial filters are used near blockboundaries. FIG. 8 is conceptual diagram depicting asymmetric partialfilters at a horizontal boundary. FIG. 9 is conceptual diagram depictingasymmetric partial filters at a vertical boundary. In this approach, asshown in FIGS. 9 and 10, only available pixels (i.e., pixels within thecurrent block) are used for filtering. Filter taps outside the blockboundary are skipped. The filters in FIG. 8 and FIG. 9 are referred toas asymmetric because there are more filter taps used on one side(either the horizontal or vertical side) of the center of the filtermask then the other. As the entire filter mask is not used, the filtercoefficients may be renormalized to produce the desired results.Techniques for renormalization will be discussed in more detail below.

In Case 1 of FIG. 8, the center 221 of filter mask 220 is one row ofpixels away from a horizontal block boundary. Since filter mask 220 is a7×5 filter, one filter coefficient in the vertical direction correspondsto a pixel that is over the horizontal boundary. This filter coefficientis depicted in white, i.e., as an unfilled circle. The pixelcorresponding to the white filter coefficient is unavailable for use infiltering. As such, the filter coefficient corresponding to that pixelis not used. Likewise, in Case 2, the center 222 of filter mask 225 ison a row of pixels adjacent the horizontal block boundary. In this case,two filter coefficients correspond to pixels that are over thehorizontal boundary. As such, neither of the two white filtercoefficients in filter mask 225 is used for ALF. In both Case 1 and Case2, all black (i.e., filled circle) filter coefficients are used. Itshould be noted that filter pixels values in accordance with thisdisclosure may include filtering luminance components of the pixelvalue, filtering chrominance components of the pixel value, or filteringboth luminance and chrominance components of the pixel value.

In case 3 of FIG. 9, the center 235 of filter mask 234 is two columns ofpixels away from a vertical tile boundary. Since filter mask 234 is a7×5 filter, one filter coefficient in the horizontal directioncorresponds to a pixel that is over the vertical boundary. Again, thisfilter coefficient is depicted in white. The pixel corresponding to thewhite filter coefficient is unavailable for use in filtering. As such,the filter coefficient corresponding to that pixel is not used.Similarly, in Case 4, the center 233 of filter mask 232 is one column ofpixels away from a vertical block boundary. In this case, two filtercoefficients correspond to pixels that over the vertical boundary. Assuch, neither of the two white filter coefficients in filter mask 232 isused for ALF. In Case 5, the center 231 of filter mask 230 is on acolumn of pixels adjacent the vertical boundary. In this case, threefilter coefficients correspond to pixels that are over the verticalboundary. As such, none of the three white filter coefficients in filtermakes 230 are used for ALF. In all of Case 1, 2 or 3, all black filtercoefficients are used.

In another example, symmetric partial filters are used near blockboundaries. FIG. 10 is conceptual diagram depicting symmetric partialfilters at a horizontal boundary. FIG. 11 is conceptual diagramdepicting symmetric partial filters at a vertical boundary. As withasymmetric partial filters, in this approach, only available pixels areused for filtering. That is, the filter taps outside the block boundaryare skipped. Also, some coefficients of the filter mask that are withinthe current slice or tile are also not used, so as to retain asymmetrical filter mask.

For example, in Case 6 of FIG. 10, one filter coefficient in filter mask240 is outside the horizontal block boundary. The corresponding filtercoefficient within the horizontal boundary on the other side of thefilter mask is also not used. In this way, a symmetrical arrangement ofcoefficients in the vertical direction around the center coefficient 241is preserved. In Case 7 of FIG. 10, two filter coefficients in filtermask 242 are across the horizontal boundary. The corresponding twofilter coefficients on the other side of the center filter coefficient243 within the horizontal boundary are also not used. Similar examplesare shown in FIG. 11 for the vertical tile boundary. In case 8, onefilter coefficient corresponds to a pixel across the vertical blockboundary. This coefficient is not used, as well as another pixel at theleft side of the horizontal part of filter mask 250 to maintain symmetryaround center coefficient 251. Similar filter mask adjustments are madefor filter masks 252 and 254 in the case where two (Case 9) and three(Case 10) filter coefficients correspond to pixels across the verticalboundary. Shown in Cases 9 and 10, symmetry is maintained around centercoefficients 253 and 255, respectively.

Like the asymmetric partial filters shown in FIG. 8 and FIG. 9, theentire filter mask is not used for the symmetric partial filters.Accordingly, the filter coefficients may be renormalized. Techniques forrenormalization will be discussed in more detail below.

To reiterate, for each of the filter masks shown in FIGS. 8-12, afiltered value for the pixel corresponding to the center of the filtermask is calculated by multiplying a filter coefficient (represented by adarkened circle in the mask) to an associated pixel value, and thenadding the multiplied values together.

Renormalization of filter coefficients for symmetric and asymmetricpartial filter can be achieved in different ways. In general, therenormalization process recalculates the value of the remaining filtercoefficients in a partial filter mask, such that the total value of theremaining filter coefficients equals the total value of the originalfilter coefficients. Often, this total value is 1. Consider an examplewhere the original filter coefficients are labeled as C_(—)1, . . . ,C_N, where C is the value of a particular coefficient. Now assume thatthe C_(—)1, . . . , C_M coefficients do not have available correspondingpixels (i.e., the corresponding pixels are across a slice or tileboundary). Renormalized filter coefficients can be defined as follows:

EXAMPLE 1

Coeff_all=C _(—)1+C _(—)2+. . . +C _(—) N

Coeff_part=Coeff_all−(C _(—)1+. . . +C _(—) M)

New_coeffs C _(—) i′=C _(—) i*Coeff_all/Coeff_part, i=M+1, . . . , N

In example 1, Coeff_all represents the value of all coefficients in afilter mask summed together. Coeff_part represents the value of allcoefficients in a partial filter mask. That is, the summed value of thecoefficients corresponding to unavailable pixels (C_(—)1+. . . +C_M) aresubtracted from the sum of all possible coefficients in the filter mask(Coeff_all). New_coeffs_Ci′ represents the value of the filtercoefficients in the partial coefficients after a renormalizationprocess. In Example 1above, the value of the coefficient remaining inthe partial filter is multiplied by the total value of all possiblecoefficients in the filter mask (Coeff_all) and divided by the totalvalue of all coefficients in the partial filter mask (Coeff_part).

Example 2 below shows another technique for renormalizing filtercoefficients in a partial filter.

EXAMPLE 2

For subset of C_i, i=M+1, . . . , N, add C_k, k=1, . . . , M

For example,

a. C_(M+1)′=C_(M+1)+C _(—)1, C_(M+2)′=C_(M+2)+C _(—)3, . . . or

b. C _(—) L′=C _(—) L+(C _(—)1+C _(—)2+. . . +C _(—) M)

In this example, filter coefficients are renormalized by adding thecoefficients of skipped filter taps (C_k) to the coefficients ofnon-skipped filter taps (C_i).

In addition to all the solutions mentioned above with regard to ALF forinter layer prediction, a flag (flag3) may be signaled by a videoencoder to indicate that ALF application is restricted to intra pixelsin the base layer and/or inter-layer filtering.

In one example of the disclosure, the newly introduced flags (i.e.,flag1, flag2 and flag3) may be merged together into a single flag. Inanother example, flag1, flag2 and flag 3 may be merged into one flagwith the corresponding flag for DB (interLayerDeblockingFlag in H.264).As yet another example, flag1, flag2 and flag3 may be merged into oneflag with the constraint intra flag.

FIG. 12 is a block diagram illustrating an example of a video encoder 20that may use techniques for loop filtering in a video coding process asdescribed in this disclosure. The video encoder 20 will be described inthe context of SHEVC and HEVC coding for purposes of illustration, butwithout limitation of this disclosure as to other coding standards ormethods that may require loop filtering. The video encoder 20 mayperform intra- and inter-coding of CUs within video frames. Intra-codingrelies on spatial prediction to reduce or remove spatial redundancy invideo data within a given video frame. Inter-coding relies on temporalprediction to reduce or remove temporal redundancy between a currentframe and previously coded frames of a video sequence. In some examples,video encoder may be configured to perform inter-layer prediction.Intra-mode (I-mode) may refer to any of several spatial-based videocompression modes. Inter-modes such as uni-directional prediction(P-mode) or bi-directional prediction (B-mode) may refer to any ofseveral temporal-based video compression modes.

As shown in FIG. 12, the video encoder 20 receives a current video blockwithin a video frame to be encoded. In the example of FIG. 12, the videoencoder 20 includes a motion compensation unit 44, a motion estimationunit 42, an intra-prediction module 46, a reference frame buffer 64, asummer 50, a transform module 52, a quantization unit 54, and an entropyencoding unit 56. The transform module 52 illustrated in FIG. 12 is theunit that applies the actual transform or combinations of transform to ablock of residual data, and is not to be confused with block oftransform coefficients, which also may be referred to as a transformunit (TU) of a CU. For video block reconstruction, the video encoder 20also includes an inverse quantization unit 58, an inverse transformmodule 60, a summer 62, and a loop filter unit 43. Loop filter unit 43may comprise one or more of a deblocking filter unit, an SAO filterunit, and an ALF filter unit.

During the encoding process, the video encoder 20 receives a video frameor slice to be coded. The frame or slice may be divided into multiplevideo blocks, e.g., largest coding units (LCUs). The motion estimationunit 42 and the motion compensation unit 44 perform inter-predictivecoding of the received video block relative to one or more blocks in oneor more reference frames to provide temporal compression. Theintra-prediction module 46 may perform intra-predictive coding of thereceived video block relative to one or more neighboring blocks in thesame frame or slice as the block to be coded to provide spatialcompression.

The mode select unit 40 may select one of the coding modes, intra orinter, e.g., based on rate distortion results for each mode, andprovides the resulting intra- or inter-predicted block (e.g., aprediction unit (PU)) to the summer 50 to generate residual block dataand to the summer 62 to reconstruct the encoded block for use in areference frame. Summer 62 combines the predicted block with inversequantized, inverse transformed data from inverse transform module 60 forthe block to reconstruct the encoded block, as described in greaterdetail below. Some video frames may be designated as I-frames, where allblocks in an I-frame are encoded in an intra-prediction mode. In somecases, the intra-prediction module 46 may perform intra-predictionencoding of a block in a P- or B-frame, e.g., when motion searchperformed by the motion estimation unit 42 does not result in asufficient prediction of the block.

The motion estimation unit 42 and the motion compensation unit 44 may behighly integrated, but are illustrated separately for conceptualpurposes. Motion estimation (or motion search) is the process ofgenerating motion vectors, which estimate motion for video blocks. Amotion vector, for example, may indicate the displacement of aprediction unit in a current frame relative to a reference sample of areference frame. In some cases the reference frame may be in a differentlayer than the coded frame (e.g., in inter-layer prediction). The motionestimation unit 42 calculates a motion vector for a prediction unit ofan inter-coded frame by comparing the prediction unit to referencesamples of a reference frame stored in the reference frame buffer 64. Areference sample may be a block that is found to closely match theportion of the CU including the PU being coded in terms of pixeldifference, which may be determined by sum of absolute difference (SAD),sum of squared difference (SSD), or other difference metrics. Thereference sample may occur anywhere within a reference frame orreference slice, and not necessarily at a block (e.g., coding unit)boundary of the reference frame or slice. In some examples, thereference sample may occur at a fractional pixel position.

The motion estimation unit 42 sends the calculated motion vector to theentropy encoding unit 56 and the motion compensation unit 44. Theportion of the reference frame identified by a motion vector may bereferred to as a reference sample. The motion compensation unit 44 maycalculate a prediction value for a prediction unit of a current CU,e.g., by retrieving the reference sample identified by a motion vectorfor the PU.

The intra-prediction module 46 may intra-predict the received block, asan alternative to inter-prediction performed by the motion estimationunit 42 and the motion compensation unit 44. The intra-prediction module46 may predict the received block relative to neighboring, previouslycoded blocks, e.g., blocks above, above and to the right, above and tothe left, or to the left of the current block, assuming a left-to-right,top-to-bottom encoding order for blocks. The intra-prediction module 46may be configured with a variety of different intra-prediction modes.For example, the intra-prediction module 46 may be configured with acertain number of directional prediction modes, e.g., thirty-fivedirectional prediction modes, based on the size of the CU being encoded.

The intra-prediction module 46 may select an intra-prediction mode by,for example, calculating error values for various intra-prediction modesand selecting a mode that yields the lowest error value. Directionalprediction modes may include functions for combining values of spatiallyneighboring pixels and applying the combined values to one or more pixelpositions in a PU. Once values for all pixel positions in the PU havebeen calculated, the intra-prediction module 46 may calculate an errorvalue for the prediction mode based on pixel differences between the PUand the received block to be encoded. The intra-prediction module 46 maycontinue testing intra-prediction modes until an intra-prediction modethat yields an acceptable error value is discovered. Theintra-prediction module 46 may then send the PU to the summer 50.

The video encoder 20 forms a residual block by subtracting theprediction data calculated by the motion compensation unit 44 or theintra-prediction module 46 from the original video block being coded.The summer 50 represents the component or components that perform thissubtraction operation. The residual block may correspond to atwo-dimensional matrix of pixel difference values, where the number ofvalues in the residual block is the same as the number of pixels in thePU corresponding to the residual block. The values in the residual blockmay correspond to the differences, i.e., error, between values ofco-located pixels in the PU and in the original block to be coded. Thedifferences may be chroma or luma differences depending on the type ofblock that is coded.

The transform module 52 may form one or more transform units (TUs) fromthe residual block. The transform module 52 selects a transform fromamong a plurality of transforms. The transform may be selected based onone or more coding characteristics, such as block size, coding mode, orthe like. The transform module 52 then applies the selected transform tothe TU, producing a video block comprising a two-dimensional array oftransform coefficients. The transform module 52 may signal the selectedtransform partition in the encoded video bitstream.

The transform module 52 may send the resulting transform coefficients tothe quantization unit 54. The quantization unit 54 may then quantize thetransform coefficients. The entropy encoding unit 56 may then perform ascan of the quantized transform coefficients in the matrix according toa scanning mode. This disclosure describes the entropy encoding unit 56as performing the scan. However, it should be understood that, in otherexamples, other processing units, such as the quantization unit 54,could perform the scan.

Once the transform coefficients are scanned into the one-dimensionalarray, the entropy encoding unit 56 may apply entropy coding, such asCABAC.

To perform CABAC, the entropy encoding unit 56 may select a contextmodel to apply to a certain context to encode symbols to be transmitted.The context may relate to, for example, whether neighboring values arenon-zero or not. The entropy encoding unit 56 may also entropy encodesyntax elements, such as the signal representative of the selectedtransform. In accordance with the techniques of this disclosure, theentropy encoding unit 56 may select the context model used to encodethese syntax elements based on, for example, an intra-predictiondirection for intra-prediction modes, a scan position of the coefficientcorresponding to the syntax elements, block type, and/or transform type,among other factors used for context model selection.

Following the entropy coding by the entropy encoding unit 56, theresulting encoded video may be transmitted to another device, such asthe video decoder 30, or archived for later transmission or retrieval.

In some cases, the entropy encoding unit 56 or another unit of the videoencoder 20 may be configured to perform other coding functions, inaddition to entropy coding. For example, the entropy encoding unit 56may be configured to determine coded block pattern (CBP) values for CU'sand PU's. Also, in some cases, the entropy encoding unit 56 may performrun length coding of coefficients.

The inverse quantization unit 58 and the inverse transform module 60apply inverse quantization and inverse transformation, respectively, toreconstruct the residual block in the pixel domain, e.g., for later useas a reference block. The motion compensation unit 44 may calculate areference block by adding the residual block to a predictive block ofone of the frames of the reference frame buffer 64. The motioncompensation unit 44 may also apply one or more interpolation filters tothe reconstructed residual block to calculate sub-integer pixel valuesfor use in motion estimation. The summer 62 adds the reconstructedresidual block to the motion compensated prediction block produced bythe motion compensation unit 44 to produce a reconstructed video block.

The loop filter unit 43 may then perform loop filtering on thereconstructed blocks in accordance with the techniques described above.In examples of the disclosure, loop filter unit 43, alone or togetherwith other components of video encoder 20, may be configured to performloop filtering in a video coding process.

After filtering the pixels, using the loop filtering techniquesdescribed in this disclosure, the filtered reconstructed video block isthen stored in the reference frame buffer 64. The reconstructed videoblock may be used by the motion estimation unit 42 and the motioncompensation unit 44 as a reference block to inter-code a block in asubsequent video frame.

FIG. 13 is a block diagram illustrating an example of a video decoder30, which decodes an encoded video sequence. In the example of FIG. 13,the video decoder 30 includes an entropy decoding unit 70, a motioncompensation unit 72, an intra-prediction module 74, an inversequantization unit 76, an inverse transformation module 78, a referenceframe buffer 82, a loop filter unit 79, and a summer 80. The videodecoder 30 may, in some examples, perform a decoding pass generallyreciprocal to the encoding pass described with respect to the videoencoder 20 (see FIG. 12).

The entropy decoding unit 70 performs an entropy decoding process on theencoded bitstream to retrieve a one-dimensional array of transformcoefficients. The entropy decoding process used depends on the entropycoding used by the video encoder 20 (e.g., CABAC). The entropy codingprocess used by the encoder may be signaled in the encoded bitstream ormay be a predetermined process.

In some examples, the entropy decoding unit 70 (or the inversequantization unit 76) may scan the received values using a scanmirroring the scanning mode used by the entropy encoding unit 56 (or thequantization unit 54) of the video encoder 20. Although the scanning ofcoefficients may be performed in the inverse quantization unit 76,scanning will be described for purposes of illustration as beingperformed by the entropy decoding unit 70. In addition, although shownas separate functional units for ease of illustration, the structure andfunctionality of the entropy decoding unit 70, the inverse quantizationunit 76, and other units of the video decoder 30 may be highlyintegrated with one another.

The inverse quantization unit 76 inverse quantizes, i.e., de-quantizes,the quantized transform coefficients provided in the bitstream anddecoded by the entropy decoding unit 70. The inverse quantizationprocess may include a conventional process, e.g., similar to theprocesses proposed for HEVC or defined by the H.264 decoding standard.The inverse quantization process may include use of a quantizationparameter QP calculated by the video encoder 20 for the CU to determinea degree of quantization and, likewise, a degree of inverse quantizationthat should be applied. The inverse quantization unit 76 may inversequantize the transform coefficients either before or after thecoefficients are converted from a one-dimensional array to atwo-dimensional array.

The inverse transform module 78 applies an inverse transform to theinverse quantized transform coefficients. In some examples, the inversetransform module 78 may determine an inverse transform based onsignaling from the video encoder 20, or by inferring the transform fromone or more coding characteristics such as block size, coding mode, orthe like. In some examples, the inverse transform module 78 maydetermine a transform to apply to the current block based on a signaledtransform at the root node of a quadtree for an LCU including thecurrent block. Alternatively, the transform may be signaled at the rootof a TU quadtree for a leaf-node CU in the LCU quadtree. In someexamples, the inverse transform module 78 may apply a cascaded inversetransform, in which inverse transform module 78 applies two or moreinverse transforms to the transform coefficients of the current blockbeing decoded.

The intra-prediction module 74 may generate prediction data for acurrent block of a current frame based on a signaled intra-predictionmode and data from previously decoded blocks of the current frame.

Based on the retrieved motion prediction direction, reference frameindex, and calculated current motion vector, the motion compensationunit produces a motion compensated block for the current portion. Thesemotion compensated blocks essentially recreate the predictive block usedto produce the residual data.

The motion compensation unit 72 may produce the motion compensatedblocks, possibly performing interpolation based on interpolationfilters. Identifiers for interpolation filters to be used for motionestimation with sub-pixel precision may be included in the syntaxelements. The motion compensation unit 72 may use interpolation filtersas used by the video encoder 20 during encoding of the video block tocalculate interpolated values for sub-integer pixels of a referenceblock. The motion compensation unit 72 may determine the interpolationfilters used by the video encoder 20 according to received syntaxinformation and use the interpolation filters to produce predictiveblocks.

Additionally, the motion compensation unit 72 and the intra-predictionmodule 74, in an HEVC example, may use some of the syntax information(e.g., provided by a quadtree) to determine sizes of LCUs used to encodeframe(s) of the encoded video sequence. The motion compensation unit 72and the intra-prediction module 74 may also use syntax information todetermine split information that describes how each CU of a frame of theencoded video sequence is split (and likewise, how sub-CUs are split).The syntax information may also include modes indicating how each splitis encoded (e.g., intra- or inter-prediction, and for intra-predictionan intra-prediction encoding mode), one or more reference frames (and/orreference lists containing identifiers for the reference frames) foreach inter-encoded PU, and other information to decode the encoded videosequence.

The summer 80 combines the residual blocks with the correspondingprediction blocks generated by the motion compensation unit 72 or theintra-prediction module 74 to form decoded blocks. The loop filter unit79 then performs loop filtering in accordance with the techniquesdescribed above.

In examples of the disclosure, loop filter unit 79, alone or togetherwith other components of video decoder 30, may be configured to performloop filtering in a video coding process. Loop filtering may include oneor more of deblocking, ALF, and SAO filtering.

The decoded video blocks are then stored in the reference frame buffer82, which provides reference blocks for subsequent motion compensationfor inter-predictive coding and also produces decoded video forpresentation on a display device (such as the display device 32 of FIG.1).

FIG. 14 is a flowchart illustrating an example encoding method of thedisclosure. The methods of FIG. 14 may be implemented by video encoder20, including loop filter unit 43.

Video encoder 20 may be configured to encode a block of video dataaccording to a scalable video coding process to produce an encoded blockof video data (1401), decode the encoded block of video data to producea reconstructed block of video data (1402), limit the application of aloop filter to the reconstructed block of video data based on thecharacteristics of the scalable video coding process (1403), and loopfilter the reconstructed block of video data (1404).

In one example of the disclosure, video encoder 20 encodes the block ofvideo data in a base layer using a constrained intra-prediction process,and the loop filter is a sample adaptive offset (SAO) filter. In oneexample, limiting the application of the SAO filter comprises applyingthe SAO filter to boundary pixels of the block of video data in the casethat band offset SAO filtering is applied, and skipping applying the SAOfilter to boundary pixels of the block of video data in the case thatedge offset SAO filtering is applied and a neighboring block is notintra-coded. In another example, limiting the application of the SAOfilter comprises skipping applying the SAO filter to boundary pixels ofthe block of video data in the case that a neighboring block is notintra-coded. In another example, limiting the application of the SAOfilter comprises skipping applying the SAO filter to the block of videodata in the case that a neighboring block is not intra-coded. In anotherexample, limiting the application of the SAO filter comprises notsignaling an SAO type in the case that the SAO type uses pixels from aneighboring block that is not intra-coded.

In another example of the disclosure, video encoder 20 encodes the blockof video data using inter layer prediction, and the loop filter is asample adaptive offset (SAO) filter. In one example, limiting theapplication of the SAO filter comprises applying the SAO filter toboundary pixels of the block of video data in the case that band offsetSAO filtering is applied, and skipping applying the SAO filter toboundary pixels of the block of video data in the case that edge offsetSAO filtering is applied and a neighboring block is not intra-coded. Inanother example, limiting the application of the SAO filter comprisesskipping applying the SAO filter to boundary pixels of the block ofvideo data in the case that a neighboring block is not intra-coded. Inanother example, limiting the application of the SAO filter comprisesskipping applying the SAO filter to the block of video data in the casethat a neighboring block is not intra-coded and in the case that theblock of video data is coded using a constrained intra-predictionprocess. In another example, limiting the application of the SAO filtercomprises not signaling an SAO type in the case that the SAO type usespixels from a neighboring block that is not intra-coded.

In another example, video encoder 20 encodes the video data using interlayer prediction, and the loop filter is an adaptive loop filter (ALF).In one example, limiting the application of the ALF comprises applying apartial ALF to the block of video data in the case that a neighboringblock is not intra-coded.

FIG. 15 is a flowchart illustrating an example decoding method of thedisclosure. The methods of FIG. 15 may be implemented by video decoder30, including loop filter unit 79.

Video decoder 30 may be configured to decode a block of video dataaccording to a scalable video coding process to produce a reconstructedblock of video data (1501), limit the application of a loop filter tothe reconstructed block of video data based on the characteristics ofthe scalable video coding process (1502), and loop filter thereconstructed block of video data (1503).

In one example of the disclosure, video decoder 30 decodes the block ofvideo data in a base layer using a constrained intra-prediction process,and the loop filter is a sample adaptive offset (SAO) filter. In oneexample, limiting the application of the SAO filter comprises applyingthe SAO filter to boundary pixels of the block of video data in the casethat band offset SAO filtering is applied, and skipping applying the SAOfilter to boundary pixels of the block of video data in the case thatedge offset SAO filtering is applied and a neighboring block is notintra-coded. In another example, limiting the application of the SAOfilter comprises skipping applying the SAO filter to boundary pixels ofthe block of video data in the case that a neighboring block is notintra-coded. In another example, limiting the application of the SAOfilter comprises skipping applying the SAO filter to the block of videodata in the case that a neighboring block is not intra-coded.

In another example of the disclosure, video decoder 30 decodes the blockof video data using inter layer prediction, and the loop filter is asample adaptive offset (SAO) filter. In one example, limiting theapplication of the SAO filter comprises applying the SAO filter toboundary pixels of the block of video data in the case that band offsetSAO filtering is applied, and skipping applying the SAO filter toboundary pixels of the block of video data in the case that edge offsetSAO filtering is applied and a neighboring block is not intra-coded. Inanother example, limiting the application of the SAO filter comprisesskipping applying the SAO filter to boundary pixels of the block ofvideo data in the case that a neighboring block is not intra-coded. Inanother example, limiting the application of the SAO filter comprisesskipping applying the SAO filter to the block of video data in the casethat a neighboring block is not intra-coded and in the case that theblock of video data is coded using a constrained intra-predictionprocess.

In another example, video decoder 30 decodes the video data using interlayer prediction, and the loop filter is an adaptive loop filter (ALF).In one example, limiting the application of the ALF comprises applying apartial ALF to the block of video data in the case that a neighboringblock is not intra-coded.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over, as oneor more instructions or code, a computer-readable medium and executed bya hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transient media, but areinstead directed to non-transient, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules configured for encoding anddecoding, or incorporated in a combined codec. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of video coding, the method comprising:coding a block of video data according to a scalable video codingprocess to produce a reconstructed block of video data; and limiting theapplication of a loop filter to the reconstructed block of video databased on characteristics of the scalable video coding process.
 2. Themethod of claim 1, wherein coding the block of video data comprisescoding the block of video data in a base layer using a constrainedintra-prediction process, and wherein the loop filter is a sampleadaptive offset (SAO) filter.
 3. The method of claim 2, wherein limitingthe application of the SAO filter comprises: applying the SAO filter toboundary pixels of the reconstructed block of video data in the casethat band offset SAO filtering is applied; and skipping applying the SAOfilter to boundary pixels of the reconstructed block of video data inthe case that edge offset SAO filtering is applied and a neighboringblock is not intra-coded.
 4. The method of claim 2, wherein limiting theapplication of the SAO filter comprises: skipping applying the SAOfilter to boundary pixels of the reconstructed block of video data inthe case that a neighboring block is not intra-coded.
 5. The method ofclaim 2, wherein limiting the application of the SAO filter comprises:skipping applying the SAO filter to the reconstructed block of videodata in the case that a neighboring block is not intra-coded.
 6. Themethod of claim 2, wherein limiting the application of the SAO filtercomprises: not signaling an SAO type in the case that the SAO type usespixels from a neighboring block that is not intra-coded.
 7. The methodof claim 1, wherein the scalable video coding process includes interlayer prediction, and wherein the loop filter is a sample adaptiveoffset (SAO) filter.
 8. The method of claim 7, wherein limiting theapplication of the SAO filter comprises: applying the SAO filter toboundary pixels of the reconstructed block of video data in the casethat band offset SAO filtering is applied; and skipping applying the SAOfilter to boundary pixels of the reconstructed block of video data inthe case that edge offset SAO filtering is applied and a neighboringblock is not intra-coded.
 9. The method of claim 7, wherein limiting theapplication of the SAO filter comprises: skipping applying the SAOfilter to boundary pixels of the reconstructed block of video data inthe case that a neighboring block is not intra-coded.
 10. The method ofclaim 7, wherein limiting the application of the SAO filter comprises:skipping applying the SAO filter to the reconstructed block of videodata in the case that a neighboring block is not intra-coded and in thecase that the block of video data is coded using a constrainedintra-prediction process.
 11. The method of claim 7, wherein limitingthe application of the SAO filter comprises: not signaling an SAO typein the case that the SAO type uses pixels from a neighboring block thatis not intra-coded.
 12. The method of claim 1, wherein the scalablevideo coding process includes inter layer prediction, and wherein theloop filter is an adaptive loop filter (ALF), and wherein limiting theapplication of the ALF comprises applying a partial ALF to thereconstructed block of video data in the case that a neighboring blockis not intra-coded.
 13. The method of claim 1, wherein coding the blockof video data comprises decoding the block of video data, the methodfurther comprising: decoding the block of video data to produce areconstructed block of video data; and loop filtering the reconstructedblock of video data.
 14. The method of claim 1, wherein coding the blockof video data comprises encoding the block of video data, the methodfurther comprising: encoding the block of video data to produce anencoded block of video data; decoding the encoded block of video data toproduce a reconstructed block of video data; and loop filtering thereconstructed block of video data.
 15. An apparatus configured to codevideo data, the apparatus comprising: a video coder configured to: codea block of video data according to a scalable video coding process toproduce a reconstructed block of video data; and limit the applicationof a loop filter to the reconstructed block of video data based oncharacteristics of the scalable video coding process.
 16. The apparatusof claim 15, wherein the video coder is further configured to code theblock of video data in a base layer using a constrained intra-predictionprocess, and wherein the loop filter is a sample adaptive offset (SAO)filter.
 17. The apparatus of claim 16, wherein the video coder isconfigured to limit the application of the SAO filter by: applying theSAO filter to boundary pixels of the reconstructed block of video datain the case that band offset SAO filtering is applied; and skippingapplying the SAO filter to boundary pixels of the reconstructed block ofvideo data in the case that edge offset SAO filtering is applied and aneighboring block is not intra-coded.
 18. The apparatus of claim 16,wherein the video coder is configured to limit the application of theSAO filter by: skipping applying the SAO filter to boundary pixels ofthe reconstructed block of video data in the case that a neighboringblock is not intra-coded.
 19. The apparatus of claim 16, wherein thevideo coder is configured to limit the application of the SAO filter by:skipping applying the SAO filter to the reconstructed block of videodata in the case that a neighboring block is not intra-coded.
 20. Theapparatus of claim 16, wherein the video coder is configured to limitthe application of the SAO filter by: not signaling an SAO type in thecase that the SAO type uses pixels from a neighboring block that is notintra-coded.
 21. The apparatus of claim 15, wherein the scalable videocoding process includes inter layer prediction, and wherein the loopfilter is a sample adaptive offset (SAO) filter.
 22. The apparatus ofclaim 21, wherein the video coder is configured to limit the applicationof the SAO filter by: applying the SAO filter to boundary pixels of thereconstructed block of video data in the case that band offset SAOfiltering is applied; and skipping applying the SAO filter to boundarypixels of the reconstructed block of video data in the case that edgeoffset SAO filtering is applied and a neighboring block is notintra-coded.
 23. The apparatus of claim 21, wherein the video coder isconfigured to limit the application of the SAO filter by: skippingapplying the SAO filter to boundary pixels of the reconstructed block ofvideo data in the case that a neighboring block is not intra-coded. 24.The apparatus of claim 21, wherein the video coder is configured tolimit the application of the SAO filter by: skipping applying the SAOfilter to the reconstructed block of video data in the case that aneighboring block is not intra-coded and in the case that the block ofvideo data is coded using a constrained intra-prediction process. 25.The apparatus of claim 21, wherein the video coder is configured tolimit the application of the SAO filter by: not signaling an SAO type inthe case that the SAO type uses pixels from a neighboring block that isnot intra-coded.
 26. The apparatus of claim 15, wherein the scalablevideo coding process includes inter layer prediction, and wherein theloop filter is an adaptive loop filter (ALF), and wherein the videocoder is configured to limit the application of the ALF filter byapplying a partial ALF to the reconstructed block of video data in thecase that a neighboring block is not intra-coded.
 27. The apparatus ofclaim 15, wherein the video coder is a video decoder, the video decoderfurther configured to: decode the block of video data to produce areconstructed block of video data; and loop filter the reconstructedblock of video data.
 28. The apparatus of claim 15, wherein the videocoder is a video encoder, the video encoder further configured to:encode the block of video data to produce an encoded block of videodata; decode the encoded block of video data to produce a reconstructedblock of video data; and loop filter the reconstructed block of videodata.
 29. An apparatus configured to code video data, the apparatuscomprising: means for coding a block of video data according to ascalable video coding process to produce a reconstructed block of videodata; and means for limiting the application of a loop filter to thereconstructed block of video data based on characteristics of thescalable video coding process.
 30. A computer-readable storage mediumstoring instructions that, when executed, cause one or more processorsof a device configured to code video data to: code a block of video dataaccording to a scalable video coding process to produce a reconstructedblock of video data; and limit the application of a loop filter to thereconstructed block of video data based on characteristics of thescalable video coding process.